The present invention relates generally to data detection in an optical Partial Response Maximum Likelihood (PRML) read channel, and particularly to error correction circuitry for improving data detection by correcting errors due to a dominant error event in an optical PRML read channel.
DVD, an acronym for Digital Video Disc or Digital Versatile Disc, is a relatively new type of Compact-Disc Read-Only-Memory (CD-ROM) with a minimum capacity of approximately 4.7 gigabytes. FIG. 1 illustrates in block diagram form apparatus for recording to and reading data from DVD 22. Recording Unit 20 takes digital data mk and records it on DVD 20. (The subscript xe2x80x9ckxe2x80x9d is used throughout to indicate generally a time-variant signal and the subscript xe2x80x9cknxe2x80x9d indicates the value of a time-variant signal at a time k+n.) DVD player 24 includes Optical Pick-up Unit (OPU) 26, and an optical Partial Response Maximum Likelihood (PRML) Read Channel (Read Channel) 30. OPU 26 converts information read from DVD 22 into an analog RF signal on line 27. Read Channel 30 takes this RF signal and generates a digital signal qk. Read Channel 30 includes Automatic Gain Control (AGC) and Equalization Circuitry 32, Analog-to-Digital Converter (ADC) 34 and Viterbi Decoder 36. AGC and Equalization Circuitry 32 filters and limits the voltage magnitude of the RF signal on line 27, producing the analog signal on line 33. ADC 34 samples the analog signal on line 33 and produces a multi-bit digital signal, yk, on line 35 that represents the magnitude of the analog signal on line 33. Viterbi Decoder 36 analyzes the yk signal over several sample values and determines the most likely value represented by each sample. Viterbi Decoder 36 represents the most likely values via its output signal, qk, on line 40, which is a single bit in a Non-Return to Zero Inverted (NRZI) format. Ideally, qk should be identical to mk; however, errors prevents this.
Much of the error in qk is caused by baseline wandering. As used herein, baseline wandering refers to low frequency disturbances of a radio frequency signal. FIG. 2A illustrates an ideal input signal to ADC 34, which is free from baseline wandering. The signal graphed in FIG. 2A remains centered about a baseline, zero volts in this example, throughout the illustrated time period. FIG. 2B illustrates a second input signal to ADC 34, which is subject to baseline wandering. The illustrated input signal has no fixed baseline; i.e., it exhibits a variable DC offset. The variable DC offset of the radio frequency signal produces a time variable error in yk, the output of ADC 34. FIG. 3A is a histogram of the yk signal given an input signal to ADC 34 that is free from baseline wandering; i.e., given the signal of FIG. 2A. In the absence of baseline wandering, the histogram of the yk signal represents five distinctive sample values, 1, ⅔, 0, xe2x88x92⅔ and xe2x88x921. Baseline wandering of the signal to be sampled by ADC 34 produces a quite different histogram. FIG. 3B is a histogram of the yk signal given the input signal of FIG. 2B. FIG. 3B indicates that ADC 34 does not produce distinct sample values in the presence of baseline wandering, producing instead every sample value between approximately xe2x88x921.25 to 1.25. FIG. 3C through FIG. 3G are individual histograms for each ideal sample value. Thus, FIG. 3C is a histogram of sample values corresponding the ideal value of 1; FIG. 3D is a histogram of sample values corresponding to the ideal value of ⅔; FIG. 3E is a histogram of sample values corresponding to the ideal value of 0; FIG. 3F is a histogram of sample values corresponding to the ideal value of xe2x88x92⅔; and FIG. 3G is a histogram of sample values corresponding to the ideal value of xe2x88x921. These histograms reveal that baseline wandering destroys the one to one correspondence between ideal sample values and the values output by ADC 34. For example, FIGS. 3C and 3D indicate that a yk value of +xc2xe may be due to either an ideal sample value of either 1 or ⅔. Thus, a need exists for circuitry to correct data detection errors caused by baseline wandering.
The apparatus of the present invention corrects a data detection error caused by baseline wandering in an optical PRML read channel. The apparatus includes error detection circuitry and error correction circuitry. The error detection circuitry monitors a serial output signal from the optical PRML read channel and a first set of input signals to the optical PRML read channel to detect an error event associated with baseline wandering. The error detection circuitry deems an error event to have occurred when three conditions are satisfied. First, a bit sequence represented by the serial output signal matches a first bit sequence associated with the error event. Second, a first difference in a first set of consecutive values represented by the first set of input signals is within a first range of values associated with the error event. Third, a second difference in a second set of consecutive values of the first input signal is within a second range of values associated with the error event. The error detection circuitry responds to satisfaction of all three conditions by asserting an error signal. The error correction circuitry responds to assertion of the error signal by modifying a pair of consecutive bits represented by the serial output signal to generate a corrected output signal having a second bit sequence.